The invention relates to a method for detecting a “rotating stall” fault in a compressor which is driven by a converter-fed three-phase AC motor.
The following faults can occur when operating compressors, namely “surge”, “stall” and “rotating stall” faults. In the publication with the title “Rotating Stall —An Overview of Dresser-Rand-Experience” by J. M. Sorokes, published February 2003 by the Dresser-Rand Company, Houston, Tex., these faults are briefly explained before discussing the “rotating stall” fault in detail.
The “rotating stall” fault frequently precedes the “surge” fault. Both faults are permissible to a limited extent, but because of the vibrations which occur with the “rotating stall” fault, this gives rise to material fatigue. This “rotating stall” fault has previously only been detected in large compressors with the help of pressure sensors and/or vibration recorders. The quality and the selectivity of the detection depend on the positioning and the number of sensors and/or recorders. These pressure sensors and/or vibration recorders are laborious to install and evaluate. When using pressure sensors, problems can occur relating to the sealing of the installation points and relating to the selectivity of the state detection.
A compressor which is driven by a converter-fed three-phase AC machine is disclosed in DE 10 2004 060 206 B3. The converter used has a field-oriented closed-loop control system. According to this patent specification, a method for operating a converter-fed compressor is specified with which the duration and intensity of surge states are intended to be significantly reduced. With this method, an actual operating state of the compressor is determined from signal variables from the compressor drive converter and a stored pump limit characteristic. By using signal variables from the converter-fed three-phase machine, it can very quickly be established whether an inadmissible operating state exists. The variables torque-forming current component and the actual speed value from the drive converter are used for this purpose. A pump limit characteristic and a speed-dependent torque characteristic are also required for this method.